I've had a look at this, and thankfully it's an easy fix, but it does have to be applied to each Editor script. Unity changed the return type of the GetDisplayTitle method in Unity 2021.2 - the fix I'll be implementing is wrapping the method in a conditional like this:
#if UNITY_2021_2_OR_NEWER
public override GUIContent GetDisplayTitle()
{
return new GUIContent("Name of the effect goes here");
}
#else
public override string GetDisplayTitle()
{
return "Name of the effect goes here";
}
#endif
You'll also need to import the UnityEngine namespace, which your IDE will likely tell you to do. Hope that helps - let me know if there are still issues with any of the scripts!